The invention relates to the formation of packages for individual slices of food items, particularly processed cheese slices. More specifically, the invention is directed to the formation of food packages using new film materials in which a heated cross-seal is formed using the heat of the food item to activate a film sealant.
The packaging of processed cheese slices as practiced by cheese converters is a highly sophisticated technology which has continued to evolve over most of this century. As explained in more detail below, while heat plays a critical role in current package sealing techniques for processed cheese, it is carefully controlled using external heating devices since a temperature that is too high can have a deleterious impact on the processed cheese.
It is known to continuously package processed cheese slices in wrapped packages (see, e.g., U.S. Pat. No. 3,542,570 to Bush et. al. and U.S. Pat. No. 5,347,792 to Meli et. al. (Meli I)), including those that are hermetically sealed (see, e.g., U.S. Pat. Nos. 5,112,632 and 5,440,860, each to Meli et. al. (Meli II)). The Bush, Meli I and Meli II patents, each owned by the current assignee, are hereby incorporated by reference herein in their entirety.
Various packaging films have been used for this purpose. Known packaging films include coextruded films, such as films having a base layer of polypropylene, polyethylene or HDPE (high density polyethylene), with a sealant layer. One known sealant layer is polyethylene, for example, which may or may not have an intermediate adhesive layer, as disclosed in the Meli II patent. Coextruded xe2x80x9chot meltxe2x80x9d films are also known in which the sealant layer is activated by heat. Upon activation, the xe2x80x9chot meltxe2x80x9d sealant layer flows, allowing it to bond either to itself or to a base layer.
Coextruded xe2x80x9chot meltxe2x80x9d packaging films have been known for at least about 25 years. However, no known coextruded xe2x80x9chot meltxe2x80x9d packaging film has been provided that has a sealant layer designed to be activated by the heat of the processed cheese itself, and which results in peelable heat seals. Extruded xe2x80x9chot meltxe2x80x9d films have tended to provide seals which fail through adhesive failure, as opposed to cohesive failure, often resulting in an unsightly seal which may not be peelable. These films also typically have sealants with activation temperatures that substantially exceed 200xc2x0 F. However, it has been found that such temperatures have a detrimental impact on processed cheese. Thus, while it is known to heat packaging film using external heating devices so as to form seals (see, e.g., the Bush, Meli I and Meli II patents), it is not believed to be known to provide a process for packaging peelably-sealed processed cheese slices in which cross-seals are formed using only the heat of the cheese as the sealant activating agent.
Despite its obvious advantages, the current lack of a peelable package enclosed by heat seals activated by the heat of the processed cheese is believed mainly due to the general unavailability of suitable packaging films whose sealant layers have sufficiently low melt temperatures such that the heat of the processed cheese will activate the sealant layer. It may also be due to the relatively recent recognition of the importance of providing processed cheese slice packages completely enclosed by heat seals that are also manually peelable (1989, Meli II), a recognition which is still gaining industry appeal.
Accordingly, it is an object of the present invention to provide heat-sealed, film packaged processed cheese slices, using the heat of the cheese as the film sealant activating agent.
It is another object of the invention to provide such packaged cheese slices in film packages having peelable seals.
It is another object of the invention to provide a process that can be practiced using existing machines that are easily retrofitted or adapted for use with the present invention.
It is a further object of the invention to provide film materials that can be used to accomplish the objects just mentioned.
The following terms are used in the claims of the patent as filed and are intended to have their broadest meaning consistent with the requirements of law.
xe2x80x9cHeat sealxe2x80x9d is a seal effected through the application of heat to elevate the temperature of a film sealant, enabling the sealant to flow and causing an attraction and/or bond between the sealant and either itself or another portion of the film.
xe2x80x9cPeelablexe2x80x9d refers to a seal which may be manually broken by a consumer without rupturing or tearing of the film.
xe2x80x9cAdhesive failurexe2x80x9d means a seal failure mode in which a film sealant layer separates from a base or substrate film layer.
xe2x80x9cCohesive failurexe2x80x9d means a seal failure mode in which portions of a film sealant layer(s) separate from itself/themselves.
Where alternative meanings are possible, the broadest meaning is intended. All words used in the claims are intended to be used in the normal, customary usage of grammar and the English language.
The objects mentioned above, as well as other objects, are solved by the present invention, which overcomes disadvantages of prior art packaging methods and films, while provided new advantages not previously obtainable.
In one embodiment of the present invention, a method is provided for peelably packaging processed cheese slices using a plastic film having at least one base layer, such as polypropylene, and at least one sealant layer. First, the plastic film is formed into an elongated enclosure having an opening. Then, a longitudinal heat seal is provided that closes the opening. Hot molten processed cheese is now provided into the elongated enclosure. Then, opposing portions of the plastic film are compressed across the length of the film at cross-sealing zones formed at periodic intervals along the length of the enclosure to remove at least some of the processed cheese from the cross-sealing zones. The heat of the processed cheese activates the at least one sealant layer and the combination of the heat of the processed cheese and the compression forms heat seals at the cross-sealing zones, resulting in a package entirely enclosed by peelable heat seals. Alternatively, at least some of or all of the processed cheese can be removed from the cross-sealing zones prior to the compression step. The heat seals are preferably cooled immediately after their formation, using expedient means such as cooling water.
In a preferred embodiment, the step of compressing is accomplished using a two-step process in which the elongated film enclosure containing the processed cheese is passed through a pair of opposed flattening belts having periodic protrusions and then through a rotary crimping device. Alternatively, for example, the step of compressing may be accomplished by passing the elongated film enclosure through a pair of opposed, substantially smooth flattening belts and then through a crimping device.
In another preferred embodiment, both the longitudinal seal and the cross-seals are each hermetic, so that a hermetically sealed package entirely enclosing the processed cheese is formed.
In one embodiment, the plastic film may be extruded and be a non-oriented polyolefin-based formulation, such as polypropylene film. In other embodiments, the plastic film may also be a solution cast or calendered material.
In alternative preferred embodiments, the sealant layer may consist of one or more of the following: polybutylene; ethylene acrylic acid; metallocene polyethylenes; ethylene-octene copolymer; ethylene-alpha olefin copolymer; propylene alpha olefin copolymer; ethylene alpha olefin copolymer; Surlyn(copyright); DuPont Appeel(copyright); Montell PB(copyright); ethylene methyl acrylate copolymer; or a polyolefin or its derivatives.
In a preferred embodiment, the film sealant will have an activation temperature below 200xc2x0 F., and preferably below about 180xc2x0 F.-190xc2x0 F. While, in a preferred embodiment, the cross-seals fail through cohesive failure, in another embodiment they may fail through adhesive failure. Preferably the cross-seals, upon being peeled, provide visible tamper evidence due to their translucent or whitish appearance, for example.
In another preferred embodiment, the heat seals are peelable and have a substantially uniform peel strength in which the initial peel strength is generally comparable to the propagation peel strength. Preferably, the peel strength of the heat seals remains relatively constant over a range of temperatures such as 33xc2x0 F.-120xc2x0 F.
After or during the compression step, the processed cheese may be cooled to less than about 50xc2x0 F., and preferably about 40xc2x0 F., following or during the compression step.
A machine for packaging processed cheese using a plastic film having at least a base layer and at least one sealant layer also forms a part of the present invention. The machine includes a film former for forming the plastic film into an elongated enclosure having an opening, and a longitudinal sealing device for forming a longitudinal heat seal that closes the opening. A cheese source is provided for inserting hot molten processed cheese into the elongated film enclosure and into intimate heat transfer contact with the sealant layer within the elongated enclosure. The sealant has a seal activation temperature of less than about 200xc2x0 F., and preferably less than about 180xc2x0 F.-190xc2x0 F., and is activated by the heat of the processed cheese. The machine also includes a cross-sealing device for compressing opposing portions of the plastic film across the width of the film enclosure at cross-sealing zones formed at periodic intervals to remove at least some of the cheese from the cross-sealing zones. The heat of the cheese activates the sealant and the combination of the heat of the cheese and the compression forms cross-seals that are heat seals at the cross-sealing zones, resulting in a package entirely enclosed by peelable heat seals. The machine also may have cooling means, such as a spray of chilled water, for cooling the heat seals, including preferably cooling the cross-seals immediately after their formation.
In one embodiment, the machine includes a cross-sealing device that includes a rotary crimping device. In another embodiment, the machine has a cross-sealing device that includes a pair of opposed flattening belts having periodic protrusions, followed by a rotary crimping device. The rotary crimping device may include a plurality of opposed pairs of crimping bars, each of the pairs including a crimping bar made of metal and a crimping bar made of a resilient material. The peel strength of the heat seals may be adjustable depending upon the food item to be packaged and the film selected.
In another embodiment, the invention is a plastic film for use in providing peelably packaged food items, such as processed cheese. The plastic film has a substrate, preferably of a polyolefin-based material, and a sealant with a preferred activation temperature of less than or equal to about 200 xc2x0 F. The film is capable of forming a package entirely enclosing the food item with peelable heat seals. Preferably, the heat seals are hermetic and have an initial peel strength which is generally comparable to their propagation peel strength.
In a particularly preferred embodiment, the film is a coextruded film, the sealant consists of either Surlyn or EMA, and the peelable heat seals are broken by cohesive failure. In another preferred embodiment, the heat seals have a peel strength that ranges from at least about 100 grams to about 1000 grams per 15 mm measured along the length of the seal. Preferably, the heat seals, upon being peeled, provide visible tamper evidence.
The described processes and machines may be employed using processed cheese as the packaged food item or, alternatively, may be used to package other heated food items, as those of ordinary skill in the art will appreciate.